Balance shafts are commonly used to reduce or cancel shaking forces and/or vibrations which result from residual imbalances inherent in the design architecture of machinery with rotating parts or mechanisms, such as motors. These balance shafts are sometimes called "counterbalance" shafts.
Balance shafts are particularly valuable when operator or passenger comfort and freedom from noise and vibration-related fatigue or distraction are desired, as in the case of motor vehicles such as automobiles, motorcycles, and the like. It is also advantageous to minimize vibration from the standpoint of equipment reliability. Where vibrations are reduced, the size, mass and/or complexity of the mounting structures can often also be reliably reduced, thus potentially reducing cost.
With multicylinder motor vehicle engines, the inline four-cylinder engines and 90-degree V-6 engines are favored in automotive use today due to their space efficiency and cost. Both of these engine architectures benefit from balance shafts, although for different reasons and vibratory characteristics, and thus require distinctly different balance shaft arrangements.
Balance shafts for inline four-cylinder engines typically are paired to rotate in opposite directions at twice the engine speed. The two balance shafts cancel each other's lateral shaking forces while opposing the vertical secondary shaking forces that are typical with this type of engine. Each shaft produces a single unbalance force, which taken together with its mating shaft's unbalance force, produces a resultant vertical shaking force located centrally among the bank of cylinders. These "single unbalance" type shafts are shown, for example, in U.S. Pat. No. 4,819,505.
Other engines, such as 90-degree V-6 engines (i.e., six-cylinder engine with two sets of three cylinders spaced 90-degrees apart), produce resultant imbalance forces in the form of a crankshaft-speed rotating couple. These engines benefit from a single balance shaft with two balance "weights", or masses, on opposite sides of its axis of rotation, but spaced apart axially so as to have a dynamic imbalance providing a rotating couple. The couple produced by the balance shaft is designed to oppose or cancel that of the engine when the shaft is rotated at crankshaft speed and in the opposite direction to the crankshaft. The axial location of this "rotating couple"-type shaft relative to the engine is not critical since the output of the balance shaft is a pure couple or torque on the crankcase.
Balance shafts of both types frequently incorporate an elongated support member, or shaft, which provides a structural connection between the balance weights, in the case of the rotating couple-type shaft, or between the centrally located balance weight(s) and a driving member, in the case of the single unbalance-type shaft. The elongated support member is subjected to both torsion and bending forces, and thus must be substantial enough to fulfill structural requirements. Since the mass of the elongated support member is largely "dead weight" and has little, if any, contribution to unbalance, its mass can be reduced in applications where overall mass is a factor in product cost and/or operating efficiency. These elongated support members or shafts typically have a circular cross-section. This circular section represents a structurally inefficient distribution of material that causes the components and their support structures to be more massive and often more costly than necessary.
The room or space for placement of balance shafts in the engine is typically small or limited. Balance shafts usually are constrained to operate within specified radii, whether to clear mating parts or to enable installation. Thus, efficient material usage typically motivates a balance weight cross-sectional shape that is, except for elongated support member intersection areas, "circular segment" in shape, i.e. the area between a radius and a chord. The radius of such a shape represents the clearance boundary beyond which the balance shaft cannot extend without risk of unwanted contact. The chord represents a locus of constant contribution to unbalance within the section, placing elements of mass equidistant from the axis of rotation, with regard to the ability of the mass element to generate centrifugal force in a particular direction, i.e., when viewed from a direction normal to the desired direction of unbalance force.
Typically, the "circular segment" shape of the balance weights are constant along their lengths. This enables easy calculation of their unbalance value from a design standpoint. However, this shape also results in inefficient distribution of material in the case of a rotating couple-type shaft, thus causing components and their support structures to be more massive and thus also often more costly than necessary.